The Most Massive Ultra-Compact Dwarf Galaxy in the Virgo Cluster

TL;DR

This paper characterizes the most massive ultra-compact dwarf galaxy in the Virgo Cluster, revealing its extreme density, old stellar population, and possible tidal origin, thus providing insights into galaxy evolution.

Contribution

It reports the discovery and detailed properties of the densest known galaxy in the local universe, highlighting its potential tidal origin and significance for galaxy formation theories.

Findings

M59-UCD3 is the densest galaxy in the local universe.

It has a dynamical mass of 3.7×10^8 M_sun and an effective radius of 25 pc.

Deep imaging suggests a tidal remnant indicating a stripped galaxy origin.

Abstract

We report on the properties of the most massive ultra-compact dwarf galaxy (UCD) in the nearby Virgo Cluster of galaxies using imaging from the Next Generation Virgo Cluster Survey (NGVS) and spectroscopy from Keck/DEIMOS. This object (M59-UCD3) appears to be associated with the massive Virgo galaxy M59 (NGC 4621), has an integrated velocity dispersion of 78 km/s, a dynamical mass of $3.7\times10^8 M_\odot$, and an effective radius ($R_e$) of 25 pc. With an effective surface mass density of $9.4\times10^{10} M_\odot/kpc^2$, it is the densest galaxy in the local Universe discovered to date, surpassing the density of the luminous Virgo UCD, M60-UCD1. M59-UCD3 has a total luminosity of $M_{g'}=-14.2$ mag, and a spectral energy distribution consistent with an old (14 Gyr) stellar population with [Fe/H]=0.0 and [$\alpha$/Fe]=+0.2. We also examine deep imaging around M59 and find a broad low surface brightness stream pointing towards M59-UCD3, which may represent a tidal remnant of the UCD progenitor. This UCD, along with similar objects like M60-UCD1 and M59cO, likely represents an extreme population of tidally stripped galaxies more akin to larger and more massive compact early-type galaxies than to nuclear star clusters in present-day dwarf galaxies.